Color image forming apparatus having shiftable transfer conveyor belt and attraction assisting roller

An image forming apparatus for forming an image on an image recording medium supplied from an image recording medium supply unit, comprising:

• first and second rollers arranged separate from each other, at least one of the first and second rollers being supplied with a rotation force;
• a conveyor belt stretched between the first and second rollers for attracting the image recording medium supplied from the image recording medium supply unit onto an outer surface of a conveying portion of the conveyor belt between the first and second rollers, said conveyor belt being circulated by the first and second rollers to convey the image recording medium from one of the first and second rollers toward the other of the first and second rollers, such that:
• (i) the first roller is positioned at an upstream end of the outer surface of the conveying portion of the conveyor belt in a moving direction of the outer surface of the conveying portion of the conveyor belt, (ii) the second roller is positioned at a downstream end of the outer surface of the conveying portion of the conveyor belt in the moving direction, and (iii) the image recording medium is introduced onto the outer surface of the conveying portion of the conveyor belt at the upstream end thereof;
• a plurality of image carriers arranged side by side along the outer surface of the conveying portion of the conveyor belt, wherein at least one of the image carriers, including the one of the image carriers located nearer to the second roller than the other image carriers, is in contact with the image recording medium on the outer surface of the conveying portion of the conveyor belt;
• a plurality of toner image forming devices for forming toner images of respective predetermined different colors on the image carriers;
• a plurality of image transfer devices arranged in a space encircled by the conveyor belt such that the plurality of transfer devices are opposite to the plurality of image carriers with the conveying portion of the conveyor belt being interposed therebetween, wherein at least one of the image transfer devices which is opposite to the at least one of the image carriers which is in contact with the image recording medium transfers a given toner image from the at least one of the image carriers to the image recording medium;
• a belt moving mechanism for moving the conveyor belt between a mono-color image transfer position wherein the conveyor belt is in contact with the image carrier nearer to the second roller and a full-color image transfer position wherein the conveyor belt is in contact with all of the image carriers, wherein the belt moving mechanism includes:
• (i) a stationary support roller which is arranged in the space encircled by the conveyor belt between the image transfer device opposite to the image carrier nearer to the second roller and the image transfer device opposite to the one of the image carriers next nearer to the second roller, said stationary support roller always contacting an inner surface of the conveying portion of the conveyor belt to maintain a position of the image recording medium constant relative to the image carrier nearer to the second roller when the image recording medium on the outer surface of the conveying portion of the conveyor belt arrives at the image carrier nearer to the second roller and while the conveyor belt is in the mono-color image transfer position and the full-color image transfer position, and (ii) a swing member which is swingable on a rotational axis of the stationary support roller to move the conveyor belt selectively to one or both of the mono-color image transfer position and the full-color image transfer position;
• and
• a belt movement controller for controlling the swing movement of the swing member.

An image forming apparatus for forming an image on an image recording medium supplied from an image recording medium supply unit, comprising:

• first and second rollers arranged separate from each other, at least one of the first and second rollers being supplied with a rotation force;
• a conveyor belt stretched between the first and second rollers for attracting the image recording medium supplied from the image recording medium supply unit onto an outer surface of a conveying portion of the conveyor belt between the first and second rollers, said conveyor belt being circulated by the first and second rollers to convey the image recording medium from one of the first and second rollers toward the other of the first and second rollers, such that:
• (i) the first roller is positioned at an upstream end of the outer surface of the conveying portion of the conveyor belt in a moving direction of the outer surface of the conveying portion of the conveyor belt, (ii) the second roller is positioned at a downstream end of the outer surface of the conveying portion of the conveyor belt in the moving direction, and (iii) the image recording medium is introduced onto the outer surface of the conveying portion of the conveyor belt at the upstream end thereof;
• a plurality of image carriers arranged side by side along the outer surface of the conveying portion of the conveyor belt, wherein at least one of the image carriers, including the one of the image carriers located nearer to the second roller than the other image carriers, is in contact with the image recording medium on the outer surface of the conveying portion of the conveyor belt;
• a plurality of toner image forming devices for forming toner images of respective predetermined different colors on the image carriers;
• a plurality of image transfer devices arranged in a space encircled by the conveyor belt such that the plurality of transfer devices are opposite to the plurality of image carriers with the conveying portion of the conveyor belt being interposed therebetween, wherein at least one of the image transfer devices which is opposite to the at least one of the image carriers which is in contact with the image recording medium transfers a given toner image from the at least one of the image carriers to the image recording medium;
• a roller support mechanism for supporting the first and second rollers to fix the first and second rollers to the apparatus and to fix a positional relationship between the first roller and the image recording medium supply unit;
• a belt moving mechanism for moving the conveyor belt between a mono-color image transfer position wherein the conveyor belt is in contact with the image carrier nearer to the second roller and a full-color image transfer position wherein the conveyor belt is in contact with all of the image carriers, wherein the belt moving mechanism maintains a position of the image recording medium constant relative to the image carrier nearer to the second roller when the image recording medium on the outer surface of the conveying portion of the conveyor belt arrives at the image carrier nearer to the second roller and while the conveyor belt is in the mono-color image transfer position and the full-color image transfer position;
• and
• a belt movement controller for controlling an operation of the belt moving mechanism to selectively move the conveyor belt between the mono-color image transfer position and the full-color image transfer position.